1. Field of the Invention
The present invention relates to a backlight unit, and more particularly to a backlight unit and a liquid crystal display using the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for increasing display quality.
2. Description of the Related Art
In general, the application scope of a liquid crystal display (LCD) device has been broadening due to it characteristics of light weight, thin profile, and low power consumption. For example, LCD devices are now used in office automation equipment, televisions, computer monitors, cellular phones and other types of audio/video equipment. A LCD device is not a self luminous display device. Thus, an LCD requires an external light source, such as a backlight. An active matrix type LCD devices uses thin film transistors (TFTs) as switching elements. The active matrix type LCD device controls a transmitted amount of light from the backlight in accordance with a signal applied to the TFTs, thereby displaying a desired picture on a screen.
The backlight for the LCD device can either be a direct type or an edge type. The edge type backlight has a light source installed at the edge of one side of an LCD panel, and irradiates an incident light from the light source to the entire surface of an LCD panel using a transparent light guide panel. The direct type backlight has a plurality of light sources disposed directly under the LCD panel, and irradiates a uniform planar white light to the liquid crystal display panel.
The backlight includes a light source, such as cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), and/or light emitting diodes (LEDs). Among the CCFLs, EEFLs and LEDs light sources, LEDs have the characteristics of small size, low power consumption and stability. Accordingly, LEDs are becoming widely used as light sources in backlights for LCD devices.
FIG. 1 is an exploded perspective view of a liquid crystal display device according to the related art, and FIG. 2 is a cross-sectional view along the line I-I′ in FIG. 1. FIGS. 1 and 2 are diagrams of small to medium sized LCD devices. Small and medium sized LCD devices have sizes in the range of about 10″˜30″. As shown in FIGS. 1 and 2, an LCD device of the related art includes a liquid crystal display panel 11 for displaying a picture; and a backlight unit 12 for irradiating uniform planar light into the liquid crystal display panel 11. The liquid crystal display panel 11 displays a picture by electrically controlling liquid crystal molecules to modulate light incident from the backlight unit 12.
The backlight unit 12 includes a case 18; a reflector 17 provided within the case 18; a plurality of LEDs 19 positioned in the reflector 17; a diffusion plate 15 covering the plurality of LEDs 19 and the reflector 17; a light guide panel 16 disposed between the diffusion plate 15 and the reflector 17; reflectors 20 on the light guide panel 16; and optical sheets 14 covering the diffusion plate 15. The case 18 can be a rectangular container shape having an opening and an inner space. The reflector 17 is along side surfaces and a bottom surface of the case 18 within the inner space of the case 18. The LEDs 19 are arranged in two rows in the inner space at a central area of the case 18 and protrude through the reflector 17. The reflector 17 is disposed under the LEDs 19 in the inner space of the case 18 so as to reflect the light generated from the LEDs 19, thereby increasing the efficiency of light emission to the liquid crystal display panel 11. A light guide panel 16 is positioned within the inner space of the case 18 over the LEDs 19. Reflectors 20 are attached to the light guide panel 16 directly over the LEDs 19. The opening of the case 18 is covered by a diffusion plate 15. Light from the LEDs 19 is mixed within a portion of the inner space between the case 18, the diffusion plate 15 and the light guide panel 16.
The LEDs 19 include blue LEDs generating blue light, green LEDs generating green light, and red LEDs generating red light. The LEDs are arranged in a sequence of blue LED, green LED and then red LED in each of the rows. In the alternative, the LEDs are arranged in a sequence of green LED, blue LED and then red LED in each row.
The light guide panel 16 supports the reflector 20 and is disposed between the diffusion plate 15 and the LEDs 19 within the inner spacer of the case 18. The reflectors 20 are known as “diverters” and are attached to a rear surface of the light guide panel 16 facing the LEDs 19. The reflectors 20 respectively correspond to the LEDs 19 and divert (or reflect) light vertically emitted from the LEDs toward the reflector row 17.
The red, green and blue lights generated by the LEDs 19 are reflected by the reflector 17 and the reflectors 20 attached to the light guide panel 16 into the inner space of the case 18 and are mixed into a white light within the inner space of the case 18. The diffusion plate 15 covers the opening of the case 18, and diff-uses the white light mixed within the inner space of the case 18, thereby providing uniform white light to the liquid crystal display panel 11. The optical sheets 14 also diffuse the light from the diffusion plate 15 and control the emission direction of the uniform white light.
The liquid crystal display device of the related art has a problem in that the light guide panel 16 sags due to heat generated in the inner space of the case 18 from the LEDs 19. The light guide panel 16 sags such that the reflectors 20 are not directly over the LEDs. Thus, vertical light emitted directly from the LEDs 19 is not reflected by the reflectors 20 on the light guide panel 20, but rather is incident directly onto the light guide panel 16. If vertical light emitted directly from the LEDs 19 through the diffusion plate 15 and to the optical sheets 14 is incident onto the light guide panel 20, the incident light to the liquid crystal display panel 11 has spots of red, green and/or blue tones such that color reproduction capability of the liquid crystal display panel 11 is decreased.